1. Field of the Invention
The present invention relates to satellite receivers for television receive-only (TVRO) antennas, and in particular, the present invention relates to a method and system for varying a predetection bandwidth of a satellite receiver without physically or electrically changing the bandwidths of the intermediate frequency (IF) filters.
2. Statement of the Problem
Satellite-transmitted television is enjoying tremendous growth around the world. A satellite television system includes several geostationary satellites positioned in the Clarke belt around the earth that transmit microwave signals to a receiving antenna positioned on the ground. Usually the receiving antenna is a parabolic dish to receive a low power transmission from the satellite.
The microwave signal transmitted by the satellite is captured by the receiving antenna. The captured signal is processed through a low-noise block downconverter that converts the 3.7-4.2 Gigahertz (GHz) received signal to a lower intermediate frequency (IF) signal of 0.95-1.45 GHz. This process is called downconverting. The signal from the low-noise block downconverter is passed through one or more mixers that reduce the frequency to levels that are easier to convert to baseband television audio and video signals. The signal also passes through one or more bandpass filters that select one channel from the many channels broadcast by the satellite to the receiving antenna.
C-band satellites generally use a frequency or channel plan where each channel comprises a range of frequencies 36 MHz wide with a 2 MHz guard band at the upper and lower frequencies of the channel. The C-band is 500 MHz wide, spreading from 3.7 GHz to 4.2 GHz. A center frequency of each channel is spaced 40 MHz from an adjacent channel, giving 12 channels in a 500 MHz spectrum. A second set of channels, offset by 20 MHz from the first set of channels, is transmitted in an orthogonal polarization to give a total of twenty-four channels.
C-band satellites are positioned in geostationary orbits around the earth and are spaced from each other by two degrees in an orbital arc. Because each of these satellites transmits in the same frequency band, interference between adjacent satellites is a prevalent problem. Channel polarization is coordinated between adjacent satellites to minimize interference. This coordination means that adjacent channels broadcast by adjacent satellites will be like-polarized. For example, channels 1 and 3 are like-polarized with channel 2 of an adjacent satellite. Thus, although polarization coordination is necessary for reduced interference, it cannot elimnate interference to the receiving antenna attempting to receive only channel 2.
A figure of merit for satellite receiving stations is the carrier-to-noise ratio (C/N). C/N is improved by using larger antennas or by broadcasting the signal from the satellite at a higher power or effective isotropic radiative power (EIRP). EIRP varies from satellite to satellite and is completely uncontrollable by the satellite receiver. Large diameter receiving antenna provide adequate C/N even with weak EIRP; however, there is an increasing need and desire for smaller TVRO antennas. C/N can be improved in smaller antennas by narrowing a bandwidth of a filter used to filter the downconverted signal before it is passed to an FM detector. This bandwidth is called the predetection bandwidth.
By narrowing the predetection bandwidth, some of the signal information found at the upper and lower frequencies or "skirts" of the 36 MHz channel bandwidth is lost, but the overall signal quality is improved. This lost information results in "video truncation" or loss in quality of the video signal when it is reproduced on the television screen. One particular problem is "chroma truncation" that results in white spots on the TV screen when a high-intensity color signal is broadcast but the signal is truncated by narrow bandwidth filters in the receiving circuitry. It is desirable to narrow the predetection bandwidth as little as possible in order to provide adequate C/N with minimal loss of signal information.
One common solution to this problem is to pick a single predetection bandwidth that is a compromise chosen to optimize performance of the satellite receiver for the most popular satellites and locations on earth. A fixed predetection bandwidth does not provide optimum performance for every satellite that a user wishes to access because it cannot be adjusted for the varying EIRPs of the many satellites. This solution is often acceptable when a large-diameter antenna is used in a fixed location.
Several solutions have been proposed for providing multiple predetection bandwidths in a satellite receiver. One solution is to provide multiple discreet switchable bandpass filters. This increases cost for the satellite receiver as each of the several bandpass filters is expensive. Also, multiple filters are more expensive to assemble than is a single bandpass filter.
Another solution is to provide multiple bandwidths by filters with a bandwidth that can be varied electrically such as with a variable capacitor (VARICAP). Although allowing the satellite receiver to use a single bandpass filter, VARICAPs are expensive, and typically reduce the Q of the bandpass filter to less than that which could be achieved with single-valued components. Moreover, it is desirable to use surface acoustic wave (SAW) filters for their performance, and SAW filters cannot be electrically varied.
Another solution is to provide multiple predetection bandwidths with a bandpass filter in conjunction with selectable or tunable notch filters. The notch filters are designed to greatly attenuate signals above and below the bandpass filter center frequency. This method is commonly referred to as terrestrial interference (TI) filtering. This approach requires multiple filters (at least one bandpass filter and two tunable notch filters) and so increases the cost and complexity of the system. Moreover, the systems are usually designed to limit terrestrial interference rather than carefully control predetection bandwidth.
In many applications it is desirable to provide a small antenna. In particular, the recreational vehicle market places a premium on small antenna designs. A small antenna design will require a narrow predetection bandwidth, for example 25 MHz, when a satellite EIRP is low or adjacent satellite noise is high. However, in some circumstances even a small antenna will have adequate reception with a wider predetection bandwidth. In these circumstances, reception can be improved by increasing the predetection bandwidth to fit the particular circumstances, location, and satellite that is being accessed.
Another problem with satellite receivers is that C-band receivers are not optimized to receive KU-band satellite signals. Unlike C-band, which has a very uniform channel plan, KU-band channels vary considerably in position and bandwidth. For example, each C-band satellite transmits channels one, two, and three at center frequencies of 3720 MHz, 3740 MHz, and 3760 MHz, respectively. One KUband satellite, GE K1, transmits channel one at 11,729 GHz, channel two at 11.7585 GHz, and channel three at 11.788 GHz, respectively. Another KU-band satellite, Galaxy 7, transmits channel one at 11,720 GHz, channel two at 11.750 GHz (horizontal polarity), and channel three at 11.750 GHz (vertical polarity). The GE K1 satellite uses a channel bandwidth of 59 MHz while the Galaxy 7 uses a channel bandwidth of 50 MHz.
Thus, to fine tune a KU-band channel it is necessary to fine tune the predetection bandwidth of the satellite receiver. Conventional receivers require multiple SAW bandpass filters that are expensive. Also, it is impractical to include enough SAW bandpass filters to cover all of the available KU-band channels, so even expensive high-end receivers cannot provide optimum predetection bandwidth for all of the KU-band channels.
Thus, a need exists for a satellite receiving system and method that can provide a variable predetection bandwidth to adapt the satellite receiver to particular circumstances. Further, a satellite receiver is needed to optimize performance of small-diameter antennae, and particularly small-diameter antennas that are frequently moved to new locations. Also, a satellite receiving system with a variable predetection bandwidth is needed-that is relatively low in cost and avoids the use of redundant filter circuits that are often unused. Further, a need exists for a satellite receiver that can be easily fine-tuned to receive C-band, KU-band, and other satellite signals of various channel bandwidths.